If all else fails, but you'll incur some expense and you have to be careful...
But with an angle grinder shear off the dang bolt head, works every time
Just careful you don't grind into the spindle or the blade, also a blade bolt is like $10-$12 each.

Man, I broke to rachets trying to get some 4 yr. old blades off a grooming mower. Finally used some heat......soak the crap out of it with PB Blaster, the heat the bolt up, impact wrench, then heat it up again for a bit. Mine broke loose aftet he second heat up.

I agree, the scag is the only mower I know that u have to take the nut off the top and then the bolt drops out thru the bottom.

The good thing is I can hit the top nut with a 18v impact driver and they fall to the ground (Scag on a ramp of course). Slap a then blade on and hit it again the your ready to go. Rarely ever have to touch the bolt head on bottom to remove. Much faster than the Exmark or midmont Grasspoppers, etc.